We have shown that patients with primary pulmonary hypertension (PPH) have low levels of nitric oxide (NO), a potent vasodilator, in exhaled breath suggesting a role for NO in the pathophysiology of pulmonary hypertension. The purpose of this proposed research is to determine the mechanism(s) for low NO in PPH. Potential mechanisms that will be addressedin the experiments are inhibition of NO production by CO and enzyme kinetic abnormalities affecting the sensitivity of NO synthase (NOS) to oxygen. We have previously shown that oxygen concentrations in the physiologic range determine the rate of NO synthesis in the lung. However, PPH individuals have low NO despite receiving supplemental oxygen therapy. This suggests that NO synthases in PPH may be less sensitive to oxygen and thus require higher than normal oxygen to produce NO. Furthermore, CO can directly affect endogenous NO synthesis. In preliminary data, CO in PPH is in the range that may lead to inhibition of NOS activity and NO generation. We hypothesize that NO is decreased in PPH perhaps as a consequence of high CO levels and/or impaired NO production in response to oxygen. As an extension of this hypothesis, we also propose that effective therapies such as epoprostenol, may act in part by increasing NO levels in PPH. To test this hypothesis, we propose the following specific aims: Aim 1. Determine the levels of Carbon monoxide (CO) in relationship to NO and severity of disease in PPH. Aim 2. Study the effect of oxygen and epoprostenol on NO levels in PPH in vivo. Aim 3. Study the effect of 02 and CO on NOS expression and activity in vitro. Formal course work and education are incorporated into the training component of this proposal. The candidate will have direction from leaders in the field of lung biology and pulmonary hypertension through a mentoring group. Through the 5 year award, the candidate will develop his skills in (i) hypotheses-driven design of studies, (ii) analyses and interpretation of data, and (iii) writing of manuscripts, abstracts and presenting data at meetings. The training component of this proposal will lead to the candidate's transition to a fully independent investigator.